FCNC searches at ATLAS and CMS Loïc Valéry for the ATLAS and CMS Collaborations lvalery@cern.ch Rencontres de Moriond 2019 March 17th 2019
FCNC Introduction • F lavour C hanging N eutral C urrents • Forbidden at tree-level in SM: need more complex diagrams to achieve • Very low branching ratio in SM • BR( ) ~ 10 -15 t → qH W • BR( ) ~ 10 -14 t → qZ u, c t b, d, s f f � 2 Loïc Valéry | FCNC searches in ATLAS and CMS
FCNC Introduction • F lavour C hanging N eutral C urrents • Forbidden at tree-level in SM: need more complex diagrams to achieve • Very low branching ratio in SM • BR( ) ~ 10 -15 t → qH W • BR( ) ~ 10 -14 t → qZ u, c • Enhanced in many BSM theories t • 2HDM models (~10 -6 ) b, d, s f • Including RPV SUSY scenarios • MSSM (~10 -7 ) • Extra-dimensions (~10 -5 ) f • … ⇔ Constraints on FCNC Constraints on new phenomena � 2 Loïc Valéry | FCNC searches in ATLAS and CMS
In this talk Standard Model Loïc Valéry | FCNC searches in ATLAS and CMS � 3
In this talk g γ t t u/c u/c t → gq t → γ q Z H t t Standard Model u/c u/c t → Zq t → Hq FCNC couplings Loïc Valéry | FCNC searches in ATLAS and CMS � 3
ℓ Z ℓ t u/c t → Zq
tZq: common aspects • Clear experimental signature • 3 leptons (2 with m ll close to Z-mass) • Top single- and pair-production (SP & PP) • CMS considers both (SP & PP) • ATLAS focuses on PP Loïc Valéry | FCNC searches in ATLAS and CMS � 5
tZq: common aspects • Clear experimental signature • 3 leptons (2 with m ll close to Z-mass) • Top single- and pair-production (SP & PP) • CMS considers both (SP & PP) • ATLAS focuses on PP • Typical analysis strategy • Selection of (b)-jets from top quark decays or FCNC vertex • Event reconstruction in signal region (SR) • Background calibration in control regions (CR) • Simultaneous fit across all regions Loïc Valéry | FCNC searches in ATLAS and CMS � 5
ATLAS search JHEP 07 (2018) 176 • = 1 b-jet required in SR χ 2 • Reconstruction using a minimised variable • Used to discriminate signal/background Loïc Valéry | FCNC searches in ATLAS and CMS � 6
ATLAS search JHEP 07 (2018) 176 • = 1 b-jet required in SR χ 2 • Reconstruction using a minimised variable • Used to discriminate signal/background • Main backgrounds from ttZ and WZ events • Main systematic uncertainties: background modelling ℬ ( t → Zu ) < 1.7 (2.4) × 10 − 4 ℬ ( t → Zc ) < 2.4 (3.2) × 10 − 4 Loïc Valéry | FCNC searches in ATLAS and CMS � 6
CMS search CMS-PAS-TOP-17-017 • Dedicated selection for SP- and PP-enriched regions • = 1 b-jet for SP • 2 or 3 jet including ≥ 1 b-jet for PP • Reconstruction with BDT • Especially using jet quantities (kinematics, angular, …) • Used to discriminate signal/background in each SR (trained specifically for PP/SP) • Main backgrounds from ttZ and non-prompt leptons (NPL) • Main systematic uncertainties: NPL uncertainties ℬ ( t → Zu ) < 2.4 (1.5) × 10 − 4 ℬ ( t → Zc ) < 4.5 (3.7) × 10 − 4 Loïc Valéry | FCNC searches in ATLAS and CMS � 7
H t u/c t → Hq
Signatures • Many accessible signatures depending on Higgs boson decay W ∗ /Z ∗ ¯ b τ H H H t t b t τ W/Z u/c u/c u/c H → b ¯ b H → WW */ ZZ * H → ττ γ • Dedicated analyses for each signature H t • Combined interpretation performed by ATLAS γ u/c H → γγ Loïc Valéry | FCNC searches in ATLAS and CMS � 9
Signatures • Many accessible signatures depending on Higgs boson decay W ∗ /Z ∗ ¯ b τ H H H t t b t τ W/Z u/c u/c u/c H → b ¯ b H → WW */ ZZ * H → ττ γ • Dedicated analyses for each signature H t • Combined interpretation performed by ATLAS γ u/c H → γγ Loïc Valéry | FCNC searches in ATLAS and CMS � 9
t → H ( WW */ ZZ *) q Phys. Rev. D 98 (2018) 032002 b • Multilepton : 2 same-sign leptons / 3 leptons t W • Very pure final state ! c/ ¯ u ¯ ¯ W/Z t • Main backgrounds from ttW and non-prompt leptons : estimated from MC and data, resp. W ∗ /Z ∗ • Event reconstruction : BDTs • 2 combined BDTs • Signal vs non-prompt leptons or ttW Loïc Valéry | FCNC searches in ATLAS and CMS � 10
t → H ( WW */ ZZ *) q Phys. Rev. D 98 (2018) 032002 b • Multilepton : 2 same-sign leptons / 3 leptons t W • Very pure final state ! c/ ¯ u ¯ ¯ W/Z t • Main backgrounds from ttW and non-prompt leptons : estimated from MC and data, resp. W ∗ /Z ∗ • Event reconstruction : BDTs • 2 combined BDTs • Signal vs non-prompt leptons or ttW • Main systematic uncertainties: background modelling (statistics for DD backgrounds) ℬ ( t → Hu ) < 1.9(1.5) × 10 − 3 ℬ ( t → Hc ) < 1.6(1.5) × 10 − 3 Loïc Valéry | FCNC searches in ATLAS and CMS � 10
t → H ( ττ ) q arXiv:1812.11568 (Submitted to JHEP) b • Select events with lepton and/or hadronic taus t • Classify events depending on N had − τ W c/ ¯ u ¯ t ¯ • Main background from fake taus : data-based τ − estimate in CR τ + • Event reconstruction using kinematic fit • Using all input objects to reconstruct system • Kinematics used for signal vs back. BDT Loïc Valéry | FCNC searches in ATLAS and CMS � 11
t → H ( ττ ) q arXiv:1812.11568 (Submitted to JHEP) b • Select events with lepton and/or hadronic taus t • Classify events depending on N had − τ W c/ ¯ u ¯ t ¯ • Main background from fake taus : data-based τ − estimate in CR τ + • Event reconstruction using kinematic fit • Using all input objects to reconstruct system • Kinematics used for signal vs back. BDT • Main systematic uncertainties: fake tau modelling uncertainties ℬ ( t → Hu ) < 1.7 (2.0) × 10 − 3 ℬ ( t → Hc ) < 1.9 (2.1) × 10 − 3 Loïc Valéry | FCNC searches in ATLAS and CMS � 11
t → H ( b ¯ b ) q JHEP 06 (2018) 102 • Final state with 1-lepton and several jets / b-jets • Different selections for SP and PP • Leading background : (sometimes from HF) t + jets t ¯ • Estimated from simultaneous fit across all regions • Event reconstruction • BDT to reconstruct system (assign objects) • New BDT to separate signal vs background • Systematics : dominated by b-tagging uncertainties ℬ ( t → Hu ) < 4.7 (3.4) × 10 − 3 ℬ ( t → Hc ) < 4.7 (4.4) × 10 − 3 Loïc Valéry | FCNC searches in ATLAS and CMS � 12
t → H ( b ¯ b ) q arXiv:1812.11568 (Submitted to JHEP) b • Final state with 1-lepton and several jets / b-jets t • Main background : ttbar + HF jets W c/ ¯ u ¯ t ¯ b • Event reconstruction : using likelihood ratio discriminant based on object kinematics ¯ compatibility with signal b compatibility with background Loïc Valéry | FCNC searches in ATLAS and CMS � 13
t → H ( b ¯ b ) q arXiv:1812.11568 (Submitted to JHEP) b • Final state with 1-lepton and several jets / b-jets t • Main background : ttbar + HF jets W c/ ¯ u ¯ ¯ t b • Event reconstruction : using likelihood ratio discriminant based on object kinematics ¯ compatibility with signal b compatibility with background • Main systematic uncertainties : ttbar + HF modelling and c-jet mistagging ℬ ( t → Hu ) < 5.2 (4.9) × 10 − 3 ℬ ( t → Hc ) < 4.2 (4.0) × 10 − 3 Loïc Valéry | FCNC searches in ATLAS and CMS � 13
ATLAS Combination arXiv:1812.11568 (Submitted to JHEP) JHEP 10 (2017) 129 ℬ ( t → Hu ) < 1.2 (0.83) × 10 − 3 ℬ ( t → Hc ) < 1.1 (0.83) × 10 − 3 Loïc Valéry | FCNC searches in ATLAS and CMS � 14
Summary plot Loïc Valéry | FCNC searches in ATLAS and CMS � 15
Conclusion • Strong programme searching for FCNC processes in top sector • Investigated for several FCNC couplings and final states • Sensitivity far from SM expectations … but … • … reaching sensitivity to some BSM extensions ! • Next round of analyses, with full 13 TeV dataset will benefit from: • More data (~4 times more) • Improved analysis techniques and precision Stay tuned for the next results ! Loïc Valéry | FCNC searches in ATLAS and CMS � 16
Thank you
Contact Loïc Valéry Deutsches ATLAS Elektronen-Synchrotron lvalery@desy.de +49-40-8998 5381 www.desy.de
More references ATLAS t —> qZ (ttbar @ 13 TeV) JHEP 07 (2018) 176 March 2018 t —> Hc/u multilepton (ttbar @ 13 TeV) Phys. Rev. D 98 (2018) 032002 May 2018 t —> H(yy)q (ttbar @ 13 TeV) JHEP 10 (2017) 129 May 2017 t —> Hq (bb + combo) (ttbar @ 13 TeV) arXiv:1812.11568 (Submitted to JHEP) December 2018 t —> gq (single-top @ 8 TeV) EPJC 76 (2016) 55 August 2015 HL-LHC — t —> qZ ATL-PHYS-PUB-2019-001 January 2019 CMS t —> Hc (ttbar @ 8 TeV) JHEP 02 (2017) 079 October 2016 t —> H(bb)u (single top and ttbar @ 13 TeV) JHEP 06 (2018) 102 December 2017 t —> Zc (single-top and ttbar @ 13 TeV) CMS-PAS-TOP-17-017 November 2017 t —> Zu (single-top @ 8 TeV) JHEP 07 (2017) 003 February 2017 t —> qg (single top @ 8 TeV) JHEP 02 (2017) 028 October 2016 t —> yq (single top @ 8 TeV) JHEP 04 (2016) 035 November 2015 Loïc Valéry | FCNC searches in ATLAS and CMS � 19
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